The subject of this invention relates to metal detectors for locating subterranean metal objects. Specifically, a new type of metal detector is described which has a low cost of manufacture and is usable with a minimum amount of operator instruction or know-how.
It is known in the art that subterranean metal objects may be located by transmitting and receiving an electromagnetic wave in the vicinity of the buried metal object. As the source of the electromagnetic wave comes closer to the metal object, a distortion in the field resulting from a change in magnetic permeability is produced which may be detected. The most common means for detecting changes in the field includes operating an electronic oscillator circuit having a resonant frequency in the vicinity of objects which are to be detected. When the oscillator is brought in the proximity of metal, a change in the frequency of the oscillator is detected because of changes in permeability. The change in frequency can be sensed by comparing it to an oscillator of a fixed frequency whereby the difference between the two oscillators may be made audible to an operator of the metal detector. With this class of metal detector, it is possible to even determine the nature of the material being detected in that the direction of frequency shifting of the oscillator will indicate the type of material being detected.
One of the difficulties with using this class of metal detector is that mineralized ground interferes with the detection of subterranean metal objects. The mineralized ground provides a signal which is capable of shifting the frequency of the oscillator due to changes in permeability it provides to the field produced by the oscillator in a manner similar to that produced by a metallic object.
Another class of metal detector uses a transmit and receive scheme where an underground object such as coins, rings, or metal cable is energized with an energized field producing eddy currents in the surface of the metal object which produces an alternating field. The alternating field is detected by operating a receiver above ground, having a search coil. The operator of the search coil senses the presence or absence of the electromagnetic field produced by the energized metal object and notes the location of the object based upon the strength of the signal received.
The present invention operates on a different principle which gives it substantial immunities from the effects of mineralized ground and is simpler in operation than the transmit and receive apparatus in that only a single unit is required to make the detection.